Keypads are used in various devices, including portable telephones and personal digital assistants (‘PDAs’) for example. The discrete features that the user presses to enter data may take various forms, such as push-buttons projecting through the surface or elastic relief on the surface, for example. The number of features will depend on the application that the keypad is used for but typically is of the order of 20 to 40, for example 28 if the features form a 4 by 7 array.
The keypad generates electrical signals, which are then processed by a signal processor. Typically, the signal processor has a limited number of input pins available for each input or output device to which it is connected. This is especially the case if the signal processor is integrated with an applications processor providing application programmes for the user, which is desirable, since such a configuration offers advantages such as fast reaction times in addition to reduced package size.
Another type of data entry apparatus is a touch panel or touch screen, in which a two-dimensional surface is sensitive to the position at which a user touches the surface using a pointed object such as a stylus or his finger, for example. A touch screen may simultaneously display an image produced by the device and the device reacts to the position at which the user touches the surface relative to the displayed image. The positional sensitivity of the screen or panel to the touch is essentially continuous, unlike a keypad, the discrete features of which are discontinuous. The touch screen or panel also requires input pins at a signal processor. Touch screens and panels find wide application. However keypads also find wide application both for cost reasons, if they are used instead of touch panels, and for ergonomic reasons, since they provide a simple and easy way of entering alphanumeric data.
It is desirable to limit the overall number of input pins required at the signal processor, for example to avoid increasing its package size and die size in the case of an integrated circuit processor. It is also desirable to limit the number of different signal processors.